Use your mouse to resize emacs widows

It’s easy to split emacs windows with binding keys, C-x 2 and C-x 3,
for horizontally and vertically, respectively. But resizing it with C-u xx C-x C-}, or C-x C-^, or C-x C-v is somewhat
cumbersome. Fortunately, mouse can do this for us.

Resize horizontally: move mouse cursor to those dash line in
mode-line, and then just drag it.

Resize vertically: move mouse cursor to the area just under the
split line, in the mode-line. When it shows double horizontal arrow,
drag it.